The epidermal growth factor receptor (EGFR, Her1, ErbB1) is a principal member of the ErbB family of four structurally-related cell surface receptors with the other members being Her2 (Neu, ErbB2), Her3 (ErbB3) and Her4 (ErbB4). EGFR exerts its primary cellular functions though its intrinsic catalytic tyrosine protein kinase activity. The receptor is activated by binding with growth factor ligands, such as epidermal growth factor (EGF) and transforming growth factor-alpha (TGF-α), which transform the catalytically inactive EGFR monomer into catalytically active homo- and hetero-dimers. These catalytically active dimers then initiate intracellular tyrosine kinase activity, which leads to the autophosphorylation of specific EGFR tyrosine residues and elicits the downstream activation of signaling proteins. Subsequently, the signaling proteins initiate multiple signal transduction cascades (MAPK, Akt and JNK), which ultimately mediate the essential biological processes of cell growth, proliferation, motility and survival.
EGFR is found at abnormally high levels on the surface of many types of cancer cells and increased levels of EGFR have been associated with advanced disease, cancer spread and poor clinical prognosis. Mutations in EGFR can lead to receptor overexpression, perpetual activation or sustained hyperactivity and result in uncontrolled cell growth, i.e. cancer. Consequently, EGFR mutations have been identified in several types of malignant tumors, including metastatic lung, head and neck, colorectal and pancreatic cancers. In lung cancer, mutations mainly occur in exons 18 to 21, which encode the adenosine triphosphate (ATP)-binding pocket of the kinase domain. The most clinically relevant drug-sensitive EGFR mutations are deletions in exon 19 that eliminate a common amino acid motif (LREA) and point mutations in exon 21, which lead to a substitution of arginine for leucine at position 858 (L858R). Together, these two mutations account for nearly 85% of the EGFR mutations observed in lung cancer. Both mutations have perpetual tyrosine kinase activity and as a result they are oncogenic. Biochemical studies have demonstrated that these mutated EGFRs bind preferentially to tyrosine kinase inhibitor drugs such as erlotinib and gefitinib over adenosine triphosphate (ATP).
Erlotinib and gefitinib are oral EGFR tyrosine kinase inhibitors that are first line monotherapies for non-small cell lung cancer (NSCLC) patients having activating mutations in EGFR. Around 70% of these patients respond initially, but unfortunately they develop resistance with a median time to progression of 10-16 months. In at least 50% of these initially responsive patients, disease progression is associated with the development of a secondary mutation, T790M in exon 20 of EGFR (referred to as the gatekeeper mutation). The additional T790M mutation increases the affinity of the EGFR kinase domain for ATP, thereby reducing the inhibitory activity of ATP-competitive inhibitors like gefitinib and erlotinib.
Recently, irreversible EGFR tyrosine kinase inhibitors have been developed that effectively inhibit the kinase domain of the T790M double mutant and therefore overcome the resistance observed with reversible inhibitors in the clinic. These inhibitors possess reactive electrophilic functional groups that react with the nucleophilic thiol of an active-site cysteine. Highly selective irreversible inhibitors can be achieved by exploiting the inherent non-covalent selectivity of a given scaffold along with the location of a particular cysteine residue within the ATP binding site. The acrylamide moieties of these inhibitors both undergo a Michael reaction with Cys797 in the ATP binding site of EGFRT79OM to form a covalent bond. This covalent mechanism is thought to overcome the increase in ATP affinity of the T790M EGRF double mutant and give rise to effective inhibition. However, these inhibitors may cause various undesired toxicities. Therefore, development of new inhibitors for treatment of various EGFR-related cancers is still in high demand.